1. Field of the Invention
The invention is directed to a heat-sensitive recording sheet with a substrate, having a front side and a back side, a heat-sensitive recording layer which is arranged on the front side and includes color formers and color developers, and a backcoat formed on the back side.
2. Description of the Prior Art
A prior art recording sheet is known from DE-C-38 36 660. A backcoat of this recording sheet may be formed of water-soluble high polymers such as starch, gelatins, styrene-maleic acid anhydride copolymer hydrolysates or polyvinyl alcohol and from water-insoluble polymers such as latices. The polymers may be used by themselves or in mixtures. When this known recording sheet is used as a label, the known backcoat is supposed to prevent the unwanted influence of plasticizers that are present on a sheet on which the label is to be affixed label.
In a multi-layer, thermally printable sheet with a heat-sensitive color-forming layer on the front side of the layer carrier according to DE-C-32 07 071, a prior art barrier layer comprising water-soluble polymers and a water-repellent wax or a wax-like compound are provided on the back side. The desired effect is the prevention of damaging influences, especially through plasticizers.
It was suggested in DE-A-35 29 781 to print on the coating carrier. In prior art reference order to be able to discern the printed pattern, either the heat-sensitive recording layer must be transparent or, when printing on the back of the layer carrier, the backcoat and the respective adhesive layer must be transparent. Despite the fact that the use of pigments such as those often employed for improving the suitability of the thermal head is accordingly ruled out, a complicated sequence of individual process steps is required in a disadvantageous manner because the printing process directly follows the generation of the coating base paper, and the printing process is then followed by the further coating and laminating steps.
U.S. Pat. No. 4,593,298 discloses another prior art backcoat which has an alkali salt of a styrene-maleic acid copolymer and a polymeric latex. The aim of the known backcoat is to improve resistance to water and resistance to blocking. At the same time, the runnability should be improved and the tendency of the paper to curl should be restricted.
EP-B-0 171 810 describes yet another prior art backcoat in which the stability of the recorded image is to be improved through the use of a polyurethane emulsion which is applied in a weight of 1 to 5 g/m.sup.2 in that liquids such as plasticizers, oil, water and solvents are prevented from penetrating from the back into the color developing layer.
EP-A-0 518 552 describes a prior art heat-sensitive recording material in which the substrate is either a thermoplastic film or a synthetic paper. A coating layer applied to the back comprises at least 20% of a water-soluble polymer, e.g., polyvinyl alcohol, cellulose ether or starch. The backcoat preferably also contains water-insoluble polymers such as copolymers of vinyl acetate acrylic acid esters, methacrylic acid esters and acrylic acid esters, polyurethane resins, polyvinyl chloride resins and polyvinylidene chloride resins. The backcoat is supposed to prevent the curling which develops in the heat-sensitive recording layer. A cross-linking agent may also be added, if necessary, to improve the resistance of the backcoat to water.
Another prior art heat-sensitive recording material described in DE-A-37 20 171 which has a colored fluorescent dye composition in its recording layer is provided on the back side with a barrier layer which can be produced essentially from water-soluble polymer materials such as polyvinyl alcohol, various cellulose ethers, starches, gelatins, casein and polyvinylpyrolidone and resins dispersed in water, e.g., polystyrene emulsions. The function of the barrier layer is to protect against damaging materials such as oils and plasticizers which can lead to discoloration of the images developed in the recording layer.
DE-A-38 06 201 discloses a prior art heat-sensitive recording paper in which the paper carrier has a layer containing a cationic styrene/acrylic copolymer to prevent dust development and flocculation during calendering or super-calendering of paper. Further advantages consist in a broader contact surface with the thermal head during recording, a high recording density, an outstanding dot reproduction capacity, low fogging and low adhesion as well as prevention of soiling or discoloration at the thermal head. If required, the application of the cationic styrene/acrylic copolymers may be carried out in combination with starch, polyvinyl alcohol, a latex, pigment or dye by a size press or a coating device. This reference does not describe the suitability of the known heat-sensitive recording paper for producing labels whose back side is resistant to the influence of oils and/or plasticizers.
The surface sizing of coating base paper is provided according to JP-A-60-2397, wherein the coating base paper is used for producing heat-sensitive recording materials. Alternatively, it is suggested therein to provide a precoat beneath the heat-sensitive recording layer. In the case of surface sizing, a water-soluble copolymer can be used in addition to a styrene/methacrylate.
It has been shown in the past that the known measure of developing sufficient resistance to oils, fats, plasticizers and organic solvents are not adequate.